Suni Williams and Butch Wilmore are back on Earth after nine months stranded on the International Space Station.
The pair returned to Earth today in the SpaceX dragon capsule, which splashed down off the coast of Florida.
Both experienced astronauts, neither are new to the rigours of space travel — but after 270 days in orbit, the low-gravity environment of space will have doubt impacted their bodies.
The pair will be flown to their crew quarters at NASA's Johnson Space Center in Houston for several days of health checks, which are routine for returning astronauts, before flight surgeons approve their return home to their families.
Here's what we know about what long periods of time in space do to the human body.
How will the astronauts be feeling?
We briefly saw Williams and Wilmore smiling and waving after leaving the spacecraft today.
But British astronaut Tim Peake says the pair will probably be feeling "fairly rough".
"There might be some nausea, dizziness and vertigo, and your bones and your muscles are getting used to that loading [of Earth's gravity] as you stand up and walk around.
"Your balance is having to kind of regain and [you're having to] get a sense of where you are.
"So there will be a period of rapid adjustment in the next two or three days."
That's why the crew were helped out of the spacecraft and wheeled away on stretchers.
How long will it take them to recover?
It'll probably be two or three months until they feel "back to full strength", Peake says.
"They'll have to build up all those small stabilising muscles in your lower back, in your core that you can't exercise quite as well as you'd like to up in space," he says.
Frank Rubio, who has spent 371 days in space, says his first two or three months back on Earth were about "reincorporating" himself back into life and "rehabilitating" his body.
"You adapt incredibly quickly to being in space, but then unfortunately, the readaptation process back to Earth can sometimes be a little bit longer and more difficult," he told Time Magazine in 2023.
"And that's just, I think, because the forces of gravity and the forces at play here on Earth tend to have a stronger effect on your body."
How long did Williams and Wilmore spend in space?
About nine months.
Several other astronauts have spent even longer in space, so no particular special precautions should be needed for these two, according to NASA.
What is the longest time astronauts have spent in space?
Russian astronaut Valeri Polyakovv spent 437 continuous days on board the Mir space station.
But NASA astronaut Peggy Whitson holds the record for the most cumulative time in space at 675 days — she has been to space four times.
Rubio holds the record for the longest time spent onboard the International Space Station (ISS), having spent 371 days there.
How does being in space affect your body?
Many physiological systems gradually return to normal after returning to Earth, but there can be some lasting impacts from being in space.
When Rubio returned to Earth in 2023, he told American broadcaster ABC he needed some time to readapt to Earth's gravity.
He also said he would need to rebalance his equilibrium for things like standing upright and walking around.
And Rubio said the psychological aspect of spending so long in space was tougher than he expected.
Here's how being in space can impact an astronaut's bones, muscles and vision.
Without the tug of gravity, astronauts' bone and muscle mass deteriorate quickly in space.
The muscles that are the most affected are in the:
Without a proper diet and exercise routine, astronauts also lose muscle mass in microgravity faster than they would on Earth.
And even though astronauts undertake extensive exercise while on board the ISS to combat this, a 2021 study published in the Systematic Review journal found that it was not enough to prevent losses in muscle.
NASA has learned that without Earth's gravity affecting the human body, weight-bearing bones lose on average 1-1.5 per cent of mineral density during each month of spaceflight.
After returning to Earth, bone loss might not be completely corrected by rehabilitation. However, their risk of fracturing is not higher.
Spaceflight-induced bone loss does, however, put astronauts at risk of developing kidney stones.
That's because, as bones break down, calcium can be released into other bodily systems and solidify into kidney stones.
Research published in 2022 documented bone loss in 17 ISS astronauts in missions averaging about five and a half months.
A year after returning to Earth, the astronauts exhibited on average 2.1 per cent reduced bone mineral density of the tibia — one of the bones of the lower leg — and 1.3 per cent reduced bone strength.
Nine did not recover bone mineral density after the spaceflight.
Astronauts experience changes to their eyes and vision during spaceflight, a phenomenon known as spaceflight associated neuro-ocular syndrome (SANS).
Microgravity causes a person's blood and cerebrospinal fluid to shift toward the head, and researchers suspect that these fluid shifts are an underlying cause of SANS.
Some astronauts experience lasting impairment that may require corrective lenses.
Previous studies provide valuable information
NASA has been studying the effects of space on numerous astronauts' bodies over the years.
NASA is particularly interested in investigating how the body reacts to long-duration spaceflight as the agency plans for extended missions to the Moon and Mars.
One of the ways it has done this is via what's been called the Twins Study.
The study centred on Scott Kelly — who spent nearly a year onboard the ISS — and his identical twin brother Mark.
Scott Kelly participated in several biomedical studies onboard the space station while Mark Kelly, a retired astronaut, stayed on Earth as a control subject for comparison.
The study provided valuable data about what happened to Scott Kelly, physiologically and psychologically, compared to his brother.
For example, it found that his body mass reduced by 7 per cent during the flight.
It found his cognitive performance remained largely unchanged during his year in space.
A study published in 2023 found that astronauts who travelled to the ISS or on NASA space shuttles on missions lasting at least six months experienced expansion of their cerebral ventricles — which are the spaces in the middle of the brain containing cerebrospinal fluid.
The unknowns
There are still gaps in our knowledge about how spaceflight impacts human health.
Relatively little is known about how it affects lung function.
While it is known that space radiation elevates the risk of cancer, accelerates aging and induces cellular damage, the precise biological mechanisms remain elusive.
Research has shown that mitochondria play a central role in spaceflight-induced health effects.
The precise mechanisms of mitochondrial adaptation and dysfunction in space remain an area of active study.
Scientists also lack a comprehensive understanding of how microgravity, radiation exposure and isolation impact cognitive function, mental health and neuroplasticity — the brain's ability to change and adapt — over long durations.
